Abstract: Lysander Spooner has become one of the most influential anarchist thinkers of the nineteenth century, but the details of his transition toward anarchism are unclear. This paper explores this question. I argue that although Spooner was a natural-rights Jeffersonian prior to the Civil War, it is clear he was not yet an anarchist. His writings on the constitutionality of slavery demonstrate the seeds of anarchism, but also show his willingness to effect change through the legislative process. After the Dred Scott ruling, he became markedly more radical, but the American Civil War was the catalyst for his embrace of the anarchism for which he is known today. More specifically, I contend that Spooner’s 1864 letter to Charles Sumner is the first written expression of his anarchism, a position that was retroactively explained through his No Treason pamphlets.